Any response from Tech? Current state for me seems to be that it doesn't work with Firefox, either under Android or under Windows 10, but it does work with Microsoft's (hideous) Edge browser under Windows 10. (In case that's helpfuil to anyone else with the same problem!)

In case it helps people in the meantime (or even helps tech sort it out) can we see if it's systematically particular operating system browser combinations that do/don't work? For me, Windows 10 with Edge does work. W10 with Firefox, and Android with Firefox, does not. Can go into version details if there are people out there with different experiences of those combinations? If it is failing for everyone with Ff and W10/Android it's surprising Tech can't get to the bottom of it!